This invention relates to apparatus for continuously producing photovoltaic devices on a substrate by depositing successive semiconductor layers in each of at least two adjacent glow discharge deposition chambers. The composition of each semiconductor layer is dependent upon, inter alia, the particular process gases introduced into each of the deposition chambers and the method of forming the semiconductor layer from those process gases. More particularly, the process gases introduced into the first deposition chamber are carefully controlled and isolated from the gases introduced into the adjacent deposition chamber to provide semiconductor layers of very high quality. If all semiconductor layers are not of high quality, the overall efficiency of the semiconductor device produced from those layers suffers. It is therefore necessary to carefully monitor all steps and materials which have a bearing on the quality of the semiconductor layers produced.
In the glow discharge deposition of semiconductor films onto a substrate, process gases introduced into the dedicated deposition chambers are directed to flow between a cathode and the substrate. In this area bounded by the cathode and substrate, referred to hereinafter as the plasma region, power supplied to the cathode, forms an electrodynamic field in the plasma region which operates to disassociate the process gases into species which are then deposited onto the substrate. If the electrodynamic field is not uniform over the entire length of the cathode, the properties of the semiconductor films deposited onto the substrate will be affected accordingly. More particularly, nonuniform areas in the electrodynamic field created between the cathode and substrate result in the deposition of nonhomogeneous semiconductor layers. It is therefore a principle purpose of the present invention to substantially prevent nonhomogeneous semiconductor layers formed by nonuniform electrodynamic fields from being deposited onto the substrate.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which are, in operation, substantially equivalent to their crystalling counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques which possess (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such a technique is fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent to Crystalling Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980; and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalling semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltages from each cell may be added, thereby making the greatest use of the light energy passing through the semiconductor device.
It is of obvious commercial importance to be able to mass produce amorphous photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980, for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981, for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981, for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
Recent improvements in continuous glow discharge deposition apparatus such as (1) establishing a substantially unidirectional flow of gases from the intrinsic deposition chamber to adjacent dopant chambers through a small gas gate passageway; (2) reducing the size of those passageways by employing magnetic assemblies which urge the unlayered substrate surface toward one of the passageway walls; and (3) using a flow of inert sweep gases across the gas gate passageway substantially improved the quality of semiconductor layers produced from the deposition apparatus. However, any aspect of manufacturing which adversely affects the quality of films produced cannot be tolerated. Accordingly, it has been discovered that the homogeneity of a semiconductor layer deposited onto the substrate at the portions of the plasma region proximate the ends of the cathodes varies from the homogeneity of the semiconductor layer deposited onto the substrate inward of those end portions of the cathodes.
By way of illustration, and referring to the drawing of FIG. 3, arrow 9 indicates the direction of movement of grounded substrate 11 which is spaced above cathode 34 to define a plasma region 80 therebetween wherein process gases are disassociated into elemental forms. In the plasma region 80, two different electrodynamic fields are present. Depending upon the nature of the electrodynamic field, the reaction kinetics for the plasma discharge will vary. Accordingly, the properties of the semiconductor layer deposited in the electrodynamic field labelled "A" are different than the properties of the semiconductor layer deposited in the electrodynamic fields labelled "B". It should therefore be readily apparent that such nonuniform electrodynamic fields in the plasma region 80 cause serious problems to deposition apparatus which utilize a continuously moving substrate.
Still referring to FIG. 3, the electrodynamic field A is substantially uniform in the direction of substrate movement 9, whereas the electrodynamic fields B are substantially nonuniform. This difference in uniformity of fields is due to the fact that field A is developed in a portion of the plasma region in which the distance between the substrate and the cathode is constant, thereby promoting field lines substantially perpendicular to the plane of the substrate. In contrast thereto, fields B are developed in portions of the plasma region adjacent the ends of the cathode 34 in which the substrate-cathode distance varies, thereby promoting angled or bent field lines relative to the plane of the substrate. The result of a substrate traveling through a plasma region characterized by nonuniform electrodynamic fields is the deposition thereonto of a nonhomogeneous semiconductor layer.
It is therefore the principle object of the present invention to substantially prevent the plasma formed in the presence of nonuniform fields formed adjacent the edges of the cathodes from being deposited onto the surface of the substrate as the substrate continuously moves therepast.